Performing ultrasound ranging in the presence of ultrasound interference

ABSTRACT

A distance measuring system comprises first and second transducers, and an ultrasound ranging subsystem coupled to the first and second transducers for performing a plurality of distance measurements between the first and second transducers. The distance measurement system can have various applications, including medical applications, in which case, the first and second transducers can be mounted on a catheter. The distance measuring system further comprises a filter coupled to the ultrasound ranging subsystem for filtering ultrasound interference from the plurality of distance measurements (such as, e.g., eight), and outputting a distance based on the filtered distance measurements. The filter filters the ultrasound interference by selecting one of the plurality distance measurements, in which case, the outputted distance is the selected distance measurement. Because the ultrasound interference will typically represent itself as the shortest distance measurement, the selected distance measurement is preferably greater than the minimum distance measurement (such as, e.g., the maximum distance measurement), thereby filtering the ultrasound interference out.

FIELD OF THE INVENTION

The invention relates generally to ultrasound ranging, and moreparticularly to systems and methods for performing ultrasound ranging inthe presence of ultrasound interference.

BACKGROUND OF THE INVENTION

Ultrasound ranging is a technique for computing the distance between twoultrasound transducers. The principle of ultrasound ranging isillustrated in FIG. 1, which shows two ultrasound transducers 10,20separated by a distance. One of the ultrasound transducers is designatedas a transmitting transducer 10 and the other is designated as areceiving transducer 20. To measure the distance between the transducers10,20, the transmitting transducer 10 transmits an ultrasound pulse 25,which is detected by the receiving transducer 20. The distance, d,between the transducers 10,20 is computed as

d=vτ

where v is the velocity of the ultrasound pulse 25 in the medium betweenthe transducers 10,20 and τ is the time of flight of the ultrasoundpulse 25 in traveling from the transmitting transducer 10 to thereceiving transducer 20.

One application of ultrasound ranging is in ultrasound positionaltracking to track the position of a device within a three-dimensional(3-D) coordinate system. Referring to FIG. 2, this is accomplished bymounting one or more ranging transducers 110 on the device 115 beingtracked and providing four or more reference transducers 120-1 to 120-4that are spaced apart. In this particular example, the device 115 beingtracked is a catheter tip. The ranging transducer 110 acts as areceiving transducer and each of the reference transducers 120-1 to120-4 can act both as a receiving and transmitting transducer.

To establish the 3-D coordinate system, the reference transducers 120-1to 120-4 are sequentially excited to transmit ultrasound pulses (notshown). When each reference transducer 120-1 to 120-4 transmits anultrasound pulse, the other reference transducers 120-1 to 120-4 detectthe ultrasound pulse. The relative distances between the referencetransducers 120-1 to 120-4 are then computed by performing ultrasoundranging on each of the detected ultrasound pulses. The computeddistances are then triangulated to determine the relative positionsbetween the reference transducers 120-1 to 120-4 in 3-D space. Therelative positions between the reference transducers 120-1 to 120-4 arethen mapped onto the 3-D coordinate system to provide a reference fortracking the position of the ranging transducer 110 in the 3-Dcoordinate system.

To track the position of the ranging transducer 110, and hence thedevice 115 carrying the ranging transducer 110, in the 3-D coordinatesystem, the reference transducers 120-1 to 120-4 are sequentiallyexcited to transmit ultrasound pulses. When each of the referencetransducers 120-1 to 120-4 transmits an ultrasound pulse, the rangingtransducer 110 detects the ultrasound pulse. The distance d1-d4 betweenthe ranging transducer 110 and each of the reference transducers 120-1to 120-4 is computed by performing ultrasound ranging on each of thedetected ultrasound pulses. The computed distances are then triangulatedto determine the relative position of the ranging transducer 110 to thereference transducers 120-1 to 120-4 in 3-D space. The position of theranging transducer 110 in the 3-D coordinate system is then determinedbased on the relative position of the ranging transducer 110 to thereference transducers 120-1 to 120-4 and the known positions ofreference transducers 120-1 to 120-4 in the 3-D coordinate system.

An example of a tracking system using ultrasound ranging is the RealtimePosition Management™ (RPM) tracking system developed commercially byCardiac Pathways Corporation, now part of Boston Scientific Corp. TheRPM system uses ultrasound ranging to track the positions of medicaldevices, including reference catheters, mapping catheters and ablationcatheters.

Because ultrasound ranging relies on the transmission and detection ofultrasound pulses to measure distance, it is vulnerable to ultrasoundinterference from ultrasound sources, e.g., an ultrasound imager. Forexample, ultrasound interference may be detected by the receivingtransducer 20 and misinterpreted as an ultrasound pulse from thetransmitting transducer 10, producing an erroneous distance measurement.

Therefore, there is need for systems and methods that enable the use ofultrasound ranging in the presence of ultrasound interference.

SUMMARY OF THE INVENTION

The present inventions are directed to systems and methods that enablethe use of ultrasound measuring equipment in the presence of ultrasoundinterference.

In accordance with a first aspect of the present inventions, a distancemeasuring system comprises first and second transducers, and anultrasound ranging subsystem coupled to the first and second transducersfor performing a plurality of distance measurements between the firstand second transducers. By way of non-limiting example, the distancemeasuring system, in performing the distance measurements, comprises apulse generator coupled to the first transducer for generating andtransmitting transmit pulses to the first transducer, a thresholddetector coupled to the second transducer for detecting receive pulsesfrom the second detector, and measurement means (e.g., a digitalcounter) coupled to the pulse generator and the threshold detector. Inthis case, for each distance measurement, the measurement means triggersthe pulse generator to generate and transmit a transmit pulse to thefirst transducer, measures the elapsed time between transmission of thetransmit pulse and detection of a receive pulse by the thresholddetector, and generates the distance measurement based on the measuredelapsed time.

The distance measuring system further comprises a filter coupled to theultrasound ranging subsystem for filtering ultrasound interference fromthe plurality of distance measurements (such as, e.g., eight), andoutputting a distance based on the filtered distance measurements. Thedistance measurement system can have various applications, includingmedical applications, in which case, the first and second transducerscan be mounted on a catheter.

In the preferred embodiment, the filter filters the ultrasoundinterference by selecting one of the plurality distance measurements, inwhich case, the outputted distance is the selected distance measurement.Because the ultrasound interference will typically represent itself asthe shortest distance measurement, the selected distance measurement ispreferably greater than the minimum distance measurement (such as, e.g.,the maximum distance measurement), thereby filtering the ultrasoundinterference out.

Although the present inventions should not be so limited in its broadestaspects, the filter sequentially receives the distance measurements, andfilters the ultrasound interference from the last N distancemeasurements. In this case, the filter may filter the last N distancemeasurements by selecting one of them. So that the system is moreresponsive to movements of the transducers, the filter can compute adistance variation of the N distance measurements, and compare thedistance variation to a threshold value. The filter can then output thedistance when the distance variation exceeds the threshold value, whileoutputting the most recent distance measurement received from theultrasound ranging subsystem otherwise. In effect, the filtering is onlyaccomplished when there is ultrasound interference, thereby providingmore responsiveness to the distance measuring process. The distancevariation computation can be accomplished in a variety of ways,including taking the difference between the maximum and minimum of thelast N distance measurements, calculating the variance of the last Ndistance measurements, or calculating the second derivative of the lastN distance measurements.

In accordance with a second aspect of the present inventions, a methodfor measuring the distance between two transducers comprises performinga plurality of distance measurements (e.g., eight) between thetransducers. For example, the distance measurement can comprise excitingone of the transducers to transmit an ultrasound pulse, and measuringthe time for the ultrasound pulse to reach the other transducer.

The method further comprises filtering ultrasound interference from theplurality of distance measurements, and outputting a distance based onthe filtered distance measurements. The distance measurements can befiltered by, e.g., selecting one of the plurality distance measurements,in which case, the outputted distance will be the selected distancemeasurement. The selected distance measurement is preferably more thanthe minimum distance measurement, such as, e.g., the maximum distancemeasurement.

Although the present inventions should not be some limited in itsbroadest aspects, the distance measurements are sequentially received,and the ultrasound interference is filtered from the last N distancemeasurements. In this case, the last N distance measurements can befiltered by selecting one of them. So that the method is more responsiveto movements of the transducers, a distance variation of the N distancemeasurements can be computed and compared to a threshold value. Thedistance can then be outputted when the distance variation exceeds thethreshold value, while the most recent distance measurement receivedfrom the ultrasound ranging subsystem can be outputted otherwise,thereby providing for a more responsive method. The distance variationcomputation can be accomplished in a variety of ways, including takingthe difference between the maximum and minimum of the last N distancemeasurements, calculating the variance of the last N distancemeasurements, or calculating the second derivative of the last Ndistance measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a diagram illustrating the principle of ultrasound rangingbetween two transducers.

FIG. 2 is a diagram illustrating ultrasound positional tracking usingultrasound ranging.

FIG. 3 is a functional diagram of an ultrasound ranging system.

FIG. 4 is a timeline of a transmit pulse and a received pulse used tomeasure the time of flight of an ultrasound pulse.

FIG. 5 is a diagram of the ultrasound ranging system of FIG. 3 in anenvironment containing a source of ultrasound interference.

FIG. 6 is a timeline depicting the arrival of ultrasound interferencebetween the transmit pulse and the receive pulse.

FIG. 7 is a functional diagram of the ultrasound ranging system furthercomprising a distance filter according to one embodiment of theinvention.

FIG. 8 is a flowchart illustrating the operation of a distance filteraccording to another embodiment of the invention.

FIG. 9 is a functional block diagram of an ultrasound ranging systemcomprising multiple transmitting transducers according to anotherembodiment of the invention.

FIG. 10 is a functional block diagram of an ultrasound positionaltracking system according to still another embodiment of the invention.

FIG. 11 illustrates examples of a medical catheter and a referencecatheter that can be used with the system of FIG. 10.

FIG. 12 illustrates an exemplary display image of the system of FIG. 10.

FIG. 13 is a graph illustrating the probability of a distance error withand without the filter of the invention as a function of distancebetween two transducers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a block diagram of an ultrasound ranging system 310 formeasuring the distance between transducers 10,20. The ranging system 310generally includes a pulse generator 320 coupled to the transmittingtransducer 10, a threshold detector coupled 330 to the receivingtransducer 20, and distance circuitry 340 coupled to the thresholddetector 330. The pulse generator 320 may generate voltage pulses havinga frequency of, e.g., 600 KHz. The threshold detector 330 detectssignals from the receiving transducer 20 that are above a thresholdlevel, e.g., a voltage level. The ranging system 310 further includescontrol and timing circuitry 350 coupled to the pulse generator 320, anda distance counter 360 coupled to the control and timing circuitry 350and the distance circuitry 340. The distance counter 360 may be adigital counter driven by a clock. For ease of discussion, only onetransmitting transducer 10 is shown. The more typical case of multipletransmitting transducers will be discussed later.

To measure the distance between the transducers 10,20, the control andtiming circuitry 350 triggers the pulse generator 320 to generate andtransmit a transmit pulse to the transmitting transducer 10. Thetransmitting transducer 10 converts the transmit pulse into anultrasound pulse and transmits the ultrasound pulse 25. The control andtiming circuitry 350 also triggers the distance counter 360 to begincounting from zero at the transmit time of the transmit pulse. Therunning count value of the distance counter 360 provides a measure oftime from the transmission of the transmit pulse.

After the ultrasound pulse 25 has been transmitted, the receivingtransducer 20 receives the ultrasound pulse and converts the ultrasoundpulse into a receive pulse, which is detected by the threshold detector330. Upon detection of the receive pulse, the distance circuitry 340reads the current count value from the distance counter 360. The readcount value indicates the elapsed time between the transmission of thetransmit pulse and the detection of the receive pulse, which correspondsto the time of flight, τ, of the ultrasound pulse 25 between thetransducers 10,20. This is illustrated in FIG. 4, which shows a timelineof the transmit pulse and the receive pulse. The read count value alsoprovides a distance measurement between the transducers 10,20. This isbecause the distance, d, between the transducers 10,20 is proportionalto the time of flight, τ, of the ultrasound pulse 25, by

d=vτ

where v is the velocity of the ultrasound pulse 25. The distancecircuitry 340 outputs the read count value as a distance measurementbetween the transducers 10,20.

In one embodiment, the distance circuitry 340 listens for the receivepulse within a time window, e.g., 100 μsec, after the transmit pulse hasbeen transmitted. The time window may begin immediately or shortly afterthe transmit pulse has been transmitted. In determining the time ofdetection of the receive pulse, the distance circuitry 340 interpretsthe first signal that the threshold detector 330 detects within the timewindow as the receive pulse.

The operation of the ultrasound ranging system 310 in an environmentcontaining ultrasound interference will now be described with referenceto FIG. 5, in which a source 510 of ultrasound interference 525 isintroduced. The source of ultrasound interference may be, e.g., anultrasound imaging transducer. In the following discussion, it will beassumed that the ultrasound interference 525 is large enough inamplitude to be detected by the threshold detector 330.

When the ultrasound interference reaches the receiving transducer 20before transmission of the transmit pulse, the distance circuitry 340ignores the ultrasound interference. This is because the distancecircuitry 340 does not listen for the receive pulse until after thetransmit pulse has been transmitted.

FIG. 6 is a timeline illustrating the case in which the ultrasoundinterference reaches the receiving transducer 20 between the transmitpulse and the receive pulse. When this occurs, the threshold detector330 detects the ultrasound interference first. As a result, the distancecircuitry 340 misinterprets the detected ultrasound interference as thereceive pulse. This causes the distance measurement 350 unit toprematurely read the count value from the distance counter 360. In thiscase, the read count value indicates the time between the transmit pulseand the detection of the ultrasound interference. As a result, the readcount value corresponds to an erroneous time of flight, τ_(err), that isshorter than the actually time of flight, τ, of the ultrasound pulse, asillustrated in FIG. 6. This in turn causes the distance circuitry 340 tooutput a distance measurement that is shorter than the actual distancebetween the transducers 10,20.

When the ultrasound interference reaches the receiving transducer 20after the receive pulse, the distance measurement circuitry 340 ignoresthe ultrasound interference. This is because the threshold detector 330detects the receive pulse first. As a result, the distance circuitry 340correctly reads the count value at the detection of the receive pulse.

Of the three cases discussed above, the only case in which the rangingsystem 310 is affected by the ultrasound interference is when theultrasound interference reaches the receiving transducer 20 between thetransmit pulse and the receive pulse. In this case, the distancecircuitry 340 outputs a distance measurement that is shorter than theactual distance between the transducers 10,20. Thus, ultrasoundinterference causes the ranging system 310 to measure distances that aretoo short. The invention exploits this property to filter out erroneousdistance measurements caused by ultrasound interference, as explainedfurther below.

FIG. 7 illustrates an embodiment of a system 710 for measuring thedistance between transducers 10,20 further including a distance filter715 coupled to the distance circuitry 340. In the preferred embodiment,the distance filter 715 is implemented in software, but can beimplemented in firmware or hardware as well. In this embodiment, thecontrol and timing circuitry 350 may continuously initiate distancemeasurements between the transducers 10,20 at regular intervals (e.g.,once every 13 ms for a RPM system). Each time the control and timingcircuitry 340 initiates a distance measurement, the distance circuitry340 outputs a distance measurement to the distance filter 715. Thefilter 715 takes the last N distance measurements outputted by thedistance circuitry 340, and outputs the maximum of the N distancemeasurements, where N is a positive integer (e.g., N=8).

The operation of the distance filter 715 can be represented as

y(n)=max[x(n), x(n−1), . . . , x(n−N)]

where y(n) is the most recent output of the distance filter, x(n) is themost recent distance measurement from the distance circuitry 340, andmax[ ] is a function that takes the maximum of the last N distancemeasurements from the distance circuitry 340 starting with the mostrecent distance measurement x(n).

The distance filter 715 filters out erroneous distance measurementscaused by ultrasound interference. This is because ultrasoundinterference causes the distance circuitry 340 to output distancemeasurements that are shorter than the actual distance between thetransducers 10,20. As a result, correct distance measurements outputtedby the distance circuitry 340 will be larger than erroneous distancemeasurements caused by ultrasound interference. Therefore, when at leastone of the last N distance measurements is correct, the maximum distancemeasurement outputted by the distance filter 715 will be one of thecorrect distance measurements. The distance filter 715 only outputs adistance error when every one of the last N distance measurements fromthe distance circuitry 340 is in error.

The distance filter 715 of the invention can significantly reducedistance errors due to ultrasound interference. This can be demonstratedby assuming that the probability of a distance error from the distancecircuitry 340 due to ultrasound interference is P. In this case, theprobability that every one of the last N distance measurements is inerror is P to the Nth power. Since the distance filter 715 outputs adistance error only when every one of the last N distance measurementsis in error, the probability of a distance error from the distancefilter 715 is P to the Nth power, which can be significantly smallerthan P. For example, if P equals 77% and N=8, the probability of adistance error from the distance circuitry 340 due to ultrasoundinterference is 77%, while the probability of a distance error from thedistance filter due to ultrasound interference is 12.3%. Obviouslyincreasing N can further reduce the probability of a distance error fromthe distance filter 715 due to ultrasound interference. However,increasing N may increase another type of distance error, as explainedfurther below.

For the case in which the control and timing circuitry 350 initiatesdistance measurements at regular intervals (e.g., once every 13milliseconds), the distance filter 715 outputs the maximum distancemeasurement over a finite measurement time window, M, of

M=NΔt

where Δt is the time between adjacent distance measurements and N is thenumber of distance measurements considered. For example, when Δt=13 msand N=8, the measurement time window is 104 ms. In order for thedistance filter 715 to provide a good approximation of the currentdistance between the transducers 10,20, the distance between thetransducers 10,20 should remain relatively stable within the measurementwindow. This ensures that the maximum distance measurement within themeasurement window provides a good approximation of the current distancebetween the transducers 10,20. When the distance between the transducers10,20 varies within measurement window, the maximum distance may nolonger closely approximate the current distance between the transducers10,20. For example, when the distance between the transducers 10,20decreases within the measurement window, the maximum distance will tendto be larger than the current distance between the transducers 10,20.

Therefore, there is tradeoff in increasing N. Increasing N decreasesdistance error due to ultrasound interference, but also increasesdistance error due to distance variation within the measurement windowby increasing the size of the measurement window.

One way to reduce error caused by distance variation within themeasurement window is to shorten the measurement window. This may beaccomplished without decreasing N, e.g., by increasing the transmit rateof the system 710 in order to provide more distance measurements withina shorter period of time. This way, distance error due to distancevariation can be reduced without increasing distance error due toultrasound interference.

In another embodiment, the distance filter 715 computes a distancevariation within the measurement window, and compares the computeddistance variation to a threshold value. The distance filter 715 outputsthe maximum distance measurement only when the distance variation withinthe measurement window exceeds the threshold value. Otherwise, thefilter 715 outputs the most recent distance measurement from thedistance circuitry 340.

The threshold value may be determined by the maximum that the distancebetween the transducers 10,20 can change within the measurement windowdue to movement between the transducers 10,20. For example, when thereceiving transducer 20 is mounted on a catheter (not shown), themaximum change in distance may be determined by the maximum distancethat a physician navigates the catheter within the measurement window.The threshold value may be represented as

threshold=Nδ

where δ represents the maximum change in distance between adjacentdistance measurements and N is the number of distance measurementsconsidered.

The operation of the distance filter 715 according to this embodimentwill now be described with reference to FIG. 8. In step 810, thedistance filter 715 computes a distance variation for the last Ndistance measurements from the distance circuitry 340. The distancevariation may be computed as the difference between the maximum andminimum of the last N distance measurements. This is represented as

range(n)=max[x(n),x(n−1), . . . ,x(n−N)]−min[x(n), x(n−1), . . .,x(n−N)]

where range(n) is the distance variation for the last N distancemeasurements, and x(n) is the most recent distance measurement from thedistance circuitry 340. Other measures of distance variation may beused, such as computing the variance of the last N distancemeasurements. As another example, the second derivative of the last Ndistance measurements, which can be used to differentiate between rapidcatheter movement and random interference can be taken. This is becauserapid catheter movement, in the absence of interference, produces adistance measurement with a small second derivative, whereas randominterference produces a distance measurement with a relatively largesecond derivative.

In step 820, the distance filter 715 compares the distance variation tothe threshold value. If the distance variation is above the thresholdvalue, the distance filter 715, at step 830, outputs the maximum of thelast N distance measurements from the distance circuitry 340. Otherwise,at step 840, the distance filter 715 outputs the most recent distancemeasurement from the distance circuitry 340. The distance filter 715repeats the steps in FIG. 8 for each subsequent distance measurement itreceives from the distance circuitry 340.

Therefore, the distance filter 715 according to this embodiment reducesthe unwanted side effects of increasing N by only outputting the maximumdistance measurement when the computed distance variation exceeds thethreshold value. In other words, the distance filter 715 only appliesits maximum filtering function when the distance variation is mostlikely due to ultrasound interference, and not due to movement betweenthe transducers 10,20.

In the discussion above, the systems had one transmitting transducer.Many applications, such as ultrasound positional tracking, requiremultiple transmitting transducers. FIG. 9 illustrates an embodiment ofthe system 905, further including multiple transmitting transducers 10-1to 10-n, where each of the transmitting transducers is coupled to apulse generator 920-1 to 920-n. In this embodiment, the control andtiming circuitry 950 sequentially initiates distance measurementsbetween the receiving transducer 20 and each of the transmittingtransducers 10-1 to 10-n.

In operation, the control and timing circuitry 950 sequentially triggerseach one of the pulse generators 920-1 to 920-n to generate and transmita transmit pulse to its respective transmitting transducer 10-1 to 10-ncausing the transmitting transducer 10-1 to 10-n to transmit anultrasound pulse (not shown). The transmit pulses may be spaced apart intime, e.g., 1 ms, so they do not interfere within one another.

For each transmit pulse, the control and timing circuitry 950 triggersthe distance counter 360 to begin counting from zero at the transmittime of the transmit pulse. The control and timing circuitry 950 mayalso send data to the distance circuitry 340 identifying thecorresponding transmitting transducer 10-1 to 10-n. After the transmitpulse has been transmitted, the distance circuitry 340 listens for areceive pulse within a time window, e.g., 100 μs. Upon detection of thereceive pulse by the threshold detector 330, the distance circuitry 340reads the current count value from the distance counter 360, whichprovides a distance measurement between the receiving transducer 20 andthe corresponding transmitting transducer 10-1 to 10-n. The distancecircuitry outputs the distance measurement to the distance filter 915.The distance circuitry 340 may also output data identifying thetransmitting transducer 10-1 to 10-n corresponding to the distancemeasurement.

The distance filter 915 sequentially receives distance measurementscorresponding to the distance between the ranging transducer 20 and eachof the transmitting transducers 10-1 to 10-n from the distance circuitry340. For each received distance measurement, the filter 915 identitiesthe corresponding transmitting transducer 10-1 to 10-n. This may beaccomplished several ways. For example, for each distance measurement,the control and timing circuitry 950 and/or the distance circuitry 340may send data to the filter 915 identifying the correspondingtransmitting transducer 10-1 to 10-n. Alternatively, the filter 915 maydetermine the identity of the corresponding transmitting transducer 10-1to 10-2 according to the order in which it receives a sequence ofdistance measurements from the distance circuitry 340. For example, thefilter 915 may assume that the first distance measurement in thesequence corresponds to transmitting transducer 10-1, the seconddistance measurement corresponds to transmitting transducer 10-2, and soforth.

In one embodiment, the distance filter 915 outputs a maximum distancefor each of the transmitting transducers 10-1 to 10-n. In determiningthe maximum distance for each transmitting transducer 10-1 to 10-n, thefilter 915 takes the maximum of the last N distance measurements forthat particular transmitting transducer 10-1 to 10-n from the distancecircuitry 340.

In another embodiment, the 915 filter also computes a distance variationfor each transmitting transducer 10-1 to 10-n by computing a variationin the last N distance measurements for that particular transmittingtransducer 10-1 to 10-n. The filter 915 may then compare the distancevariation for each transmitting transducer 10-1 to 10-n to a thresholdvalue. If the distance variation for a particular transmittingtransducer 10-1 to 10-n is below the threshold value, then the filter915 outputs the most recent distance measurement for that transmittingtransducer 10-1 to 10-n. If the distance variation for a particulartransmitting transducer 10-1 to 10-n is above the threshold value, thenthe filter 915 outputs the maximum of the last N distance measurementsfor that transmitting transducer 10-1 to 10-n.

Preferably, each time the filter 915 outputs a filtered distancemeasurement it includes data identifying the corresponding transmittingtransducer 10-1 to 10-n. If implemented in software, the filter 915 ispreferably embodied in several software modules—one for eachtransmitting transducer.

The invention is particularly well suited for use in ultrasoundpositional tracking systems to track the position of one or more devices(e.g., catheter). FIG. 10 illustrates an ultrasound positional trackingsystem 1005 according to an embodiment of the invention. The system 1005includes two or more ranging transducers 1020-1 to 1020-m mounted on themedical device being tracked (not shown), and four or more referencetransducers 1010-1 to 1010-n. Each of the ranging transducers 1020-1 to1020-m acts as a receiving transducer and each of the referencetransducers 1010-1 and 1010-n can act both as a receiving andtransmitting transducer.

FIG. 11 illustrates exemplary devices on which the ranging transducers1020-1 to 1020-m and the reference transducers 1010-1 to 1010-n can bemounted. Three ranging transducers 1020-1 to 1020-3 are mounted on adistal portion of a catheter 1110 for performing medical and/or mappingprocedures within the body. The catheter 1110 may be a mapping catheter,an ablation catheter, or the like. In this example, the rangingtransducers 1020-1 to 1020-m take the form of annular ultrasoundtransducers. Also illustrated in FIG. 11 are four reference transducers1010-1 to 1010-4 mounted on a distal portion of a reference catheter1120.

Referring back to FIG. 10, the system 1005 also includes a distancemeasurement subsystem 1015-1 to 1015-(n+m) coupled to each of theranging transducers 1020-1 to 1020-m and reference transducers 1010-1 to1010-n, and a pulse generator 1025-1 to 1025-n coupled to each of thereference transducers 1010-1 to 1010-n. Each distance subsystem 1015-1to 1015-(n+m) includes a threshold detector 1030-1 to 1030-(n+m),distance circuitry 1035-1 to 1035-(n+m) and a distance filter 1040-1 to1040-(n+m) according to the invention for filtering out distance errorsfrom the respective distance circuitry 1035-1 to 1035-(n+m). Notably, ifimplemented in software, the number of software modules for the distancefilter 1040 will equal the product of the number of ranging transducers1020 and the number of reference transducers 1010, i.e., m×n filtermodules. The system 1005 further includes control and timing circuitry1050 coupled to each of the pulse generators 1025-1 to 1025-n and adistance counter 1060 coupled to control and timing circuitry 1050 andeach of the distance measurement subsystems 1015-1 to 1015-(n+m). Thesystem 1005 also includes a triangulation circuitry 1070 fortriangulating the positions of the ranging transducers 1020-1 to 1020-mand the reference transducers 1010-1 to 1010-n, a display imageprocessor 1075 coupled to the triangulation circuitry, and a display1080 coupled to the display image processor 1075. In the preferredembodiment, the triangulation circuitry 1070 is implemented in software,but can be implemented in firmware or hardware as well.

In operation, the control and timing circuitry 1050 sequentiallytriggers each one of the pulse generators 1025-1 to 1025-n to generateand transmit a transmit pulse to its respective reference transducer1010-1 and 1010-n causing the reference transducer 1010-1 and 1010-n totransmit an ultrasound pulse. The transmit pulses may be spaced apart intime, e.g., 1 ms, so they do not interfere within one another.

For each transmit pulse, the control and timing circuitry 1050 triggersthe distance counter 1060 to begin counting from zero at the transmittime of the transmit pulse. After the transmit pulse has beentransmitted, each distance measurement subsystem 1015-1 to 1015-nlistens for a receive pulse at its respective transducer 1020-1 to1020-m and 1010-1 to 1010-n. Upon detection of a receive pulse by itsrespective threshold detector 1030-1 to 1030-(n+m), each distancecircuitry 1035-1 to 1035-(n+m) reads the current count value from thedistance counter, and outputs the read count value as a distancemeasurement to the respective distance filter 1040-1 to 1040-(n+m). Eachdistance filter 1040-1 to 1040-(n+m) filters out erroneous distancemeasurements from its respective distance circuitry 1035-1 to 1035-(n+m)due to ultrasound interference, and outputs the filtered distancemeasurements to the triangulation circuitry 1070. Each distance filter1040-1 to 1040-(n+m), preferably, includes data identifying thecorresponding receiving and transmitting transducer for each filtereddistance measurement.

For each transmit pulse, the triangulation circuitry 1070 receivesdistance measurements between the reference transducer 1010-1 to 1010-ncorresponding to the transmit pulse and each of the other referencetransducers 1010-1 to 1010-n. The triangulation circuitry 1070 alsoreceives distance measurements between the reference transducer 1010-1to 1010-n corresponding to the transmit pulse and each of the rangingtransducers 1020-1 to 1020-m.

The triangulation circuitry 1070 computes the relative positions betweenthe reference transducers 1010-1 to 1010-n in 3-D space by triangulatingthe distance measurements between the references transducers 1010-1 to1010-n. The triangulation circuitry 1070 then maps the relativepositions between the reference transducers 1010-1 to 1010-n onto the3-D coordinate system to provide a reference for tracking the positionsof the ranging transducers 1020-1 to 1020-m in the 3-D coordinatesystem. The triangulation circuitry 1070 may employ any one of a numberof mapping procedures as long as the mapping procedure preserves therelative positions between the reference transducers 1010-1 to 1010-n.

To track the positions of the ranging transducers 1020-1 to 1020-mwithin the 3-D coordinate system, the triangulation circuitry 1070triangulates the relative positions of the ranging transducers 1020-1 to1020-m to the reference transducers 1010-1 to 1010-n by triangulatingthe distance measurements between the ranging traducers 1020-1 to 1020-mand the reference traducers 1010-1 to 1010-n. The triangulationcircuitry 1070 then determines the positions of the ranging transducers1020-1 to 1020-m in the 3-D coordinate system based on the relativepositions of the ranging transducers 1020-1 to 1020-m to the referencetransducers 1010-1 to 1010-n and the known positions of referencetransducers 1010-1 to 1010-n in the 3-D coordinate system.

Those skilled in the art will appreciate that the filters 1040-1 to1040-(n+m) may be integrated in the triangulation circuitry 1070. Thismay be done, e.g., by modifying software in the triangulation circuitry1070 to perform the filtering functions according to the invention. Inthis case, the distance measurements from each distance circuitry 1040-1to 1040-(n+m) may be outputted directly to the triangulation circuitry1070, which performs the filtering functions according to the inventionbefore triangulating the positions of the transducers.

The triangulation circuitry 1070 outputs the positions of the rangingtransducers 1020-1 to 1020-m and the reference transducers 1010-1 to1010-n in the 3-D coordinate system to the display image processor 1075.The display image processor 1075 generates an image showing the positionand orientation of the device being tracked in graphical form. Thedisplay image processor 1075 may do this by plotting the positions ofthe ranging transducers 1020-1 to 1020-m in the 3-D coordinate systemand reconstructing a graphical representation of the device onto theplotted positions based on a pre-programmed graphical model of thedevice. The graphical model may include information on the relativepositions of the ranging transducers on the device. The image may alsoshow the position and orientation of the reference catheter 1120 (shownin FIG. 11) in graphical form. The display image processor 1075 may dothis by plotting the positions of the reference transducers 1010-1 to1010-n in the 3-D coordinate system and reconstructing a graphicalrepresentation of the reference catheter 1120 onto the plottedpositions. The display image processor 1075 outputs the image to thedisplay 1080, which displays the image to a physician.

FIG. 12 shows an exemplary image 1210 in which the device being trackedis a medical catheter 1110 (shown in FIG. 11). The image 1210 includesgraphical reconstructions of the medical catheter 1110 and the referencecatheter 1120. The graphical reconstruction of the medical catheter 1110is positioned and orientated in the image 1210 based on the trackedpositions of the ranging transducers 1020-1 to 1020-3 in the 3-Dcoordinate system. Similarly, the graphical reconstruction of thereference catheter 1120 is positioned and orientated in the image 1210based on the tracked positions of the reference transducers 1010-1 to1010-4 in the 3-D coordinate system. The 3-D coordinate 1215 system mayor may not be shown in the image 1210.

Even though one device was tracked in the above example, the ultrasoundpositional system 1005 may be used to track multiple devices equippedwith ranging transducers. In addition, the display 1080 may display theposition of anatomical landmarks in the 3-D coordinate system. This maybe done, e.g., by positioning a mapping catheter equipped with rangingtransducers at an anatomical landmark and recording the position of theanatomical landmark in the 3-D coordinate system based on the positionof the mapping catheter. The position of the anatomical landmark in the3-D coordinate system may then be displayed and labeled on the display1080. This enables a physician to more precisely guide devices withinthe body by referencing their tracked position on the display 1080 tothe position of the anatomical landmark on the display 1080.

The display 1080 may also display a computer representation of bodytissue in the 3-D coordinate system. This may be done, e.g., by moving amapping catheter equipped with ranging transducers to differentpositions on the surface of the body tissue and recording thesepositions in the 3-D coordinate system. The image display processor 1075may then reconstruct the computer representation of the body tissue inthe 3-D coordinate system, e.g., by fitting an anatomical shell onto therecorded positions. The computer representation of the body tissue maythen be displayed on the display 1080. This enables a physician to moreprecisely guide devices within the body by referencing their trackedposition on the display 1080 to the computer representation of the bodytissue on the display 1080. Additional details on this graphicalreconstruction technique can be found in U.S. patent application Ser.No. 09/128,304 to Willis et al. entitled “A dynamically alterablethree-dimensional graphical model of a body region”, which isincorporated by reference.

An advantage of the ultrasound tracking system 1005 according to theinvention is that the distance filters 1040-1 to 1040-(n+m) enable thetracking system 1005 to more reliably operate in an environmentcontaining ultrasound interference. This is accomplished by filteringout distance errors due to ultrasound interference, thereby improvingthe accuracy of the distance measurements used to triangulate thepositions of the ranging transducers 1020-1 to 1020-n and the referencetransducers 1010-1 to 1010-n.

The invention is especially useful for using ultrasound tracking systemsconcurrently with ultrasound imaging. One advantage of using anultrasound tracking system concurrently with ultrasound imaging is thatit allows a physician to track the position of a device within a portionof the body while at the same time imaging the portion of the body usinga ultrasound imager to provide additional information. Another advantageis that it allows the use of an ultrasound tracking system to track theposition of a device having an ultrasound imager, e.g., an ultrasoundimaging catheter.

The usefulness of the invention in using an ultrasound ranging systemconcurrently with ultrasound imaging will now be examined.

An ultrasound imager typically images the body by transmittingultrasound pulses in the body and detecting the resulting echo pulses.The rate of transmission of the ultrasound pulses is constrained by twofactors.

1. The distance that is to be imaged. This is because there must beenough time for the ultrasound energy to travel out to and back from theobject being imaged.

2. Scattering interference. This is because the scattered energy fromone ultrasound pulse must die out before the next ultrasound pulse canbe transmitted.

The more rapidly the imaging transducer is pulsing, the higher theprobability it will cause interference at an ultrasound tracking system.The probability of ultrasound interference occurring between twotransducers of an ultrasound ranging system is

P=(d/v)/T

where d is distance between the transducers, v is the velocity ofultrasound between the transducers, and T is the time between ultrasoundpulses from the interfering ultrasound imager.

In one example, an Intracardiac Echocardiography (“ICE”) cathetercontains an ultrasound imager that transmits at a rate of 7680 Hz, whichcorresponds to a time, T, of 130 μs between ultrasound pulses from theimager. FIG. 13 is a graph showing the probability of this particularultrasound imager causing interference between two transducers of anultrasound ranging system as a function of distance between thetransducers. Without the distance filter of the invention, theprobability of a distance measurement error is P=(d/v)/T, as given bythe above equation. This is shown for v=1.5 mm/μsec and T=130 μμs by thecurve labeled “unfiltered” in FIG. 13. The value v=1.5 mm/μsec is anapproximation of the velocity of ultrasound in the body. With thedistance filter of the invention, the probability of a distancemeasurement error is significantly reduced to P to the Nth power. Thisis shown for N=8 by the curve labeled “filtered” in FIG. 13.

Those skilled in the art will appreciate that various modifications maybe made to the just described preferred embodiments without departingfrom the spirit and scope of the invention. For example, the distancefilters of the invention are not limited for use with the particularultrasound ranging systems described in the specification, and may beused with other ultrasound ranging systems susceptible to ultrasoundinterference. Therefore, the invention is not to be restricted orlimited except in accordance with the following claims and their legalequivalents.

What is claimed is:
 1. A distance measuring system, comprising: firstand second transducers; a catheter on which at least one of the firstand second transducers is mounted; an ultrasound ranging subsystemcoupled to the first and second transducers for performing a pluralityof distance measurements between the first and second transducers; and afilter coupled to the ultrasound ranging subsystem for receiving anumber of the distance measurements, comparing the number of distancemeasurements to each other, selecting one of the distance measurements,wherein the particular distance measurement selected is based on thecomparison.
 2. The system of claim 1, further comprising outputting theselected distance measurement as the distance between the first andsecond transducers.
 3. The system of claim 2, wherein the selecteddistance measurement is outputted only if it is greater than the minimumdistance measurement.
 4. The system of claim 2, wherein the selecteddistance measurement is outputted only if it is the maximum distancemeasurement.
 5. The system of claim 1, wherein the distance measurementsare straight path distance measurements between the first and secondtransducers.
 6. The system of claim 1, wherein the number of distancemeasurements is
 8. 7. The system of claim 1, wherein the ultrasoundranging subsystem further comprises: a pulse generator coupled to thefirst transducer for generating and transmitting transmit pulses to thefirst transducer; a threshold detector coupled to the second transducerfor detecting receive pulses from the second detector; and measurementmeans coupled to the pulse generator and the threshold detector;wherein, for each distance measurement, the measurement means triggersthe pulse generator to generate and transmit a transmit pulse to thefirst transducer, measures the elapsed time between transmission of thetransmit pulse and detection of a receive pulse by the thresholddetector, and generates the distance measurement based on the measuredelapsed time.
 8. The system of claim 7, wherein the measurement meanscomprises a digital counter for measuring the elapsed time.
 9. Adistance measuring system, comprising: first and second transducers; acatheter on which at least one of the first and second transducers ismounted; an ultrasound ranging subsystem coupled to the first and secondtransducers for performing a plurality of distance measurements betweenthe first and second transducers; and a filter coupled to the ultrasoundranging subsystem for sequentially receiving the distance measurements,filtering ultrasound interference from the last N distance measurementsand outputting one of the N distance measurements as the distancebetween the first and second transducers based on the filtering of thedistance measurements.
 10. The system of claim 9, wherein the filtercompares all of the N distance measurements to each other, and outputsthe distance measurement based on the distance measurement comparison.11. The system of claim 10, wherein the filter outputs the distancemeasurement only if it is greater than the minimum distance measurement.12. The system of claim 10, wherein the filter outputs the distancemeasurement only if it is the maximum distance measurement.
 13. Thesystem of claim 9, wherein N is
 8. 14. The system of claim 9, whereinthe filter computes a distance variation of the N distance measurements,compares the distance variation to a threshold value, outputs themaximum distance measurement as the distance when the distance variationexceeds the threshold value, and outputs the most recent distancemeasurement received from the ultrasound ranging subsystem as thedistance otherwise.
 15. The system of claim 14, wherein the distancevariation is the difference between the maximum and minimum of the lastN distance measurements received from the ultrasound ranging subsystem.16. The system of claim 14, wherein the distance variation is thevariance of the last N distance measurements received from theultrasound ranging system.
 17. The system of claim 14, wherein thedistance variation is the second derivative of the last N distancemeasurements received from the ultrasound ranging system.
 18. A methodof measuring the distance between two transducers at least one of whichis mounted on a catheter, comprising: performing a plurality of distancemeasurements between the transducers; receiving a number of the distancemeasurements; and comparing the number of distance measurements to eachother; and selecting one of the distance measurements, wherein theparticular distance measurement selected is based on the comparison. 19.The method of claim 18, further comprising outputting the selecteddistance measurement as the distance between the first and secondtransducers.
 20. The method of claim 19, wherein the selected distancemeasurement is outputted only if it is greater than the minimum distancemeasurement.
 21. The method of claim 19, wherein the selected distancemeasurement is outputted only if it is the maximum distance measurement.22. The method of claim 18, wherein the number of distance measurementsis
 8. 23. The method of claim 18, wherein each distance measurementfurther comprises: exciting one of the transducers to transmit anultrasound pulse; and measuring the time for the ultrasound pulse toreach the other transducer.
 24. A method of measuring the distancebetween two transducers at least one of which is mounted on a catheter,comprising: performing distance measurements between the transducers;sequentially receiving the distance measurements; filtering ultrasoundinterference from the last N distance measurements; and outputting oneof the N distance measurements as the distance between the first andsecond transducers based on the filtered distance measurements.
 25. Themethod of claim 24, further comprising comparing all of the N distancemeasurements to each other, and outputting the distance measurementbased on the distance measurement comparison.
 26. The method of claim25, wherein the distance measurement is outputted only if it is greaterthan the minimum distance measurement.
 27. The method of claim 25,wherein the distance measurement is outputted only if it is the maximumdistance measurement.
 28. The method of claim 24, wherein N=8.
 29. Themethod of claim 24, further comprising the steps of: computing adistance variation of the last N distance measurements; comparing thedistance variation to a threshold value; outputting the maximum distancemeasurement as the distance when the distance variation exceeds thethreshold value; and outputting the most recent distance measurement asthe distance otherwise.
 30. The method of claim 29, wherein computingthe distance variation comprises taking the difference between themaximum and minimum of the last N distance measurements.
 31. The methodof claim 29, wherein the step of computing the distance variationcomprises computing the variance of the last N distance measurements.32. The method of claim 24, wherein each distance measurement furthercomprises: exciting one of the transducers to transmit an ultrasoundpulse; and measuring the time for the ultrasound pulse to reach theother transducer.